22 VARIATION MERISTIC AND SUBSTANTIVE. [IXTROD. 



the question arises, for the same challenge is presented in the most 

 minute and seemingly trifling details. In the skeleton of a Diatom 

 or of a Radiolarian, the scale of a Butterfly, the sculpture on a 

 pollen-grain or on an egg-shell, in the wreaths and stars of nuclear 

 division, such patterns again and again recur, and again and again 

 the question of their significance goes unanswered. There arc- 

 many suggestions, some plausible enough, as to why the tail 

 of a Peacock is gaudy, why the coat of a pollen-grain should be 

 rough, and so forth, but the significance of patterns is untouched 

 by these. Nevertheless, repetitions arranged in pattern exist 

 throughout organ i/cd Nature, in creatures that move and in those 

 that are fixed, in the great and in the small, in the seen and in 

 the hidden, within and without, as a property or attribute of Life, 

 scarcely less universal than the function of respiration or meta- 

 bolism itself. 



Such, then, is Symmetry, a character whose presence among 

 organisms approaches to universality. 



SECTION V. 

 MERISTIC VARIATION AND SUBSTANTIVE VARIATION. 



It is to the origin and nature of Symmetry that the first 

 section of the evidence of Variation will relate. That a knowledge 

 of the modes of Variation of so universal a character is important 

 to the general study of Biology must at once be evident, but to 

 the particular problem of the nature of Specific Differences this 

 importance is immense. This special importance comes from two 

 reasons. As it is the fact first that Repetition and Symmetry are 

 among the commonest features of organized structure, so it will be 

 found next that it is by differences in those features that the various 

 forms of organisms are very commonly differentiated from each other. 

 Their forms are classified by all sorts of characters, by shape and 

 proportions, by size, by colour, by habits and the like ; but perhaps 

 almost as frequently as by any of these, by differences in number 

 of parts and by differences in the geometrical relations of the parts. 

 It is by such differences that the larger divisions, genera, families, 

 &c. are especially distinguished. In such cases of course the 

 differences in number and Symmetry do not as a rule stand 

 alone, but are generally, and perhaps always, accompanied by 

 other differences of a qualitative kind ; nevertheless, the differ- 

 ences in number and Symmetry form an integral and very definite 

 part of the total differences, so that in any consideration of the 

 nature of the processes by which the differences have arisen, 

 special regard must be had to these numerical and geometrical, 

 or, as I propose to call them, Meristic, changes. 



